The invention relates to the metal plating of plastics and, in particular, to enhancing the adhesion of metal plating to polycarbonate resins by specially conditioning and etching the resin prior to electroless metal plating of the board.
The metal plating of plastic parts is well-known to be of considerable commercial importance because the desirable characteristics of both the plastic and the metal are combined to offer the technical and aesthetic advantages of each. Thus, a part plated with a bright, metallic finish takes advantage of the economies in cost and weight afforded by substituting molded plastic parts for metal and, additionally, the plated finishes are not as susceptible to pitting and corrosion because there is no galvanic reaction between a plastic substrate and a plated metal.
The problems in plating plastics are well-known in the art and one of the most serious problems is to provide a plating which both substantially covers and is adherent to the plastic. A number of methods have been developed to improve the adhesion of the metal plating to the plastic and these methods generally employ solvents to swell and condition the plastic followed by the use of oxidants to etch the surface of the plastic prior to plating. The oxidants are generally chromic acid, sulfuric acid and mixtures thereof and acidic and alkaline permanganate solutions. These solutions are expensive and, for chromium, toxic, and all present potential safety hazards as well as environmental problems such as water pollution.
The present invention relates to both enhancing the adhesion of metal plating to polycarbonate resins and providing a smooth coating and substantially complete coverage of the resins by the plating by treating the resins before plating with a specially formulated swellant composition and, preferably, using an etchant which is environmentally safe to use and inexpensive. A particularly important process is directed to . plating polycarbonate resins which have been molded into housings for electronic equipment such as personal computers. The metal plating provides a shield against electromagnetic radiation such as radio-frequency interference emitted from electronic and radio equipment. Regulations of the Federal Communication Commission require that certain categories of electronic and computer equipment be enclosed by radio-frequency interference shields. As can be appreciated, the metal plating must substantially cover the housing and be very adherent to the plastic and not delaminate during use. A number of patents addressing the adherance problem have been granted with the patents disclosing the use of many different solvents and etchants to condition the resin before plating.
U.S. Pat. No. 3,758,332 discloses the use of chemicals such as methyl ethyl ketone, tetrahydrofuran, dioxane, pyridine, dimethylformamide, and an alcohol mixture comprising methyl ethyl ketone, ethanol and methanol as swellants and H.sub.2 SO.sub.4, H.sub.3 PO.sub.4 and CrO.sub.3 as etchants for epoxy resin. U.S. Pat. No. 4,086,128 also shows pretreatment of an epoxy resin with an organic solvent comprising alcohols, acids, esters, ketones nitriles, nitro compounds, and polyhydric compounds such as ethylene glycol, glycerine and 1, 2-propylene glycol prior to etching with hydrogen peroxide and sulfuric acid. U.S. Pat. No. 3,865,623 shows immersion of epoxy resins in an organic solvent such as dimethylformamide to render the epoxy receptive to an acid etch. U.S. Pat. No. 4,592,852 shows a solvent-etch process using a permanganate etchant. The disclosures of the above publications are hereby incorporated by reference.
Unfortunately, however, solvent compositions to swell polycarbonate resins are still needed and materials such as sulfuric acid, chromium compounds and permanganate solutions are still the primary materials used to etch plastics and the need remains for a solvent and etch system which employs materials which are less expensive, safer to use and environmentally acceptable.